Fix error message when reporting unexpected yield.

R=rossberg@chromium.org
BUG=v8:2355
TEST=webkit:fast/js/reserved-words-as-property.html

Review URL: https://codereview.chromium.org/13546004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14126 ce2b1a6d-e550-0410-aec6-3dcde31c8c00